Electroplating of metals is well known. A workpiece capable of conducting an electric current is immersed in a bath containing a solution of metallic salts. A cathodic charge is imparted to the workpiece by means of a source of direct electric current. A thin layer of metal contained in the solution is thus deposited on the surface of the workpiece. A counter-electrode or anode is required in the bath. The anode may be soluble, in which case it is usually the same type of metal as is being deposited. Theoretically, metal is dissolved from the anode at the same rate as it is deposited at the cathode. Alternately, the anode may be an insoluble type. In this case the anode material does not dissolve. The result of the anodic reaction is the generation of gas. Where metallic sulfate solutions are employed, the gas is predominantly oxygen. Where appreciable concentrations of chloride are present, chlorine gas may also evolve.
U.S. Pat. No. 4,778,572 to Brown (the disclosure of which is incorporated herein by reference) indicates that, for plating processes such as zinc and nickel using soluble anodes, the anodic current efficiency is virtually 100%, meaning that essentially no gas is evolved from the anode and all the electrical current passing through the anode results in the dissolution of metal from the anode. At the cathode, the situation is somewhat different. Typically, a small, but significant percentage of the current passing through the cathode results in the production of hydrogen gas instead of metal. Normally the cathodic current efficiency varies between 90-98% in terms of metal deposition.
A difference between anode and cathode current efficiencies causes metal to dissolve from the anode faster than it plates onto the cathode. This leads to a build up in the concentration of dissolved metal in the bath solution. Eventually the dissolved metal concentration rises to the point where it is deleterious to the plating process and solution must be decanted from the plating bath and replaced with water to reduce the concentration to an acceptable level.
Coincidental with the rise in metal concentration in the electroplating solution is an increase in pH. The pH of the solution is a critical factor in maintaining satisfactory performance of the electroplating process. Rising pH is normally counteracted through regular additions of acid to maintain a constant pH in the bath.
Excess metal bearing solution decanted from an electroplating bath is environmentally hazardous and must be disposed of in an environmentally safe manner. The conventional means of disposing of such metal bearing wastes is to raise the pH with an alkali such as sodium hydroxide or lime and precipitate the metals as metal hydroxide sludge. This process is very expensive to operate and results in the generation of large quantities of solid waste which must be disposed of. Disposal of solid waste is becoming increasingly difficult and expensive. Moreover, the loss of metal value is significant.
A more attractive way to dispose of the excess solution is to electrolytically treat the solution in an electrowinning cell to recover the metal. The electrowinning cell is equipped with insoluble anodes and cathodes whereupon metal is deposited. This procedure is commonly practised in the electrolytic refining of copper. Metallic impurities build up in the copper containing electrolytes, necessitating bleeding off a certain volume of solution. Most of the copper in this solution is plated out in electrowinning cells in a procedure known in the industry as "de-copperization". The remaining solution is then disposed of. By this procedure, the copper concentration of the solution is typically reduced from more than 50 g/L to less than 10 g/L.
A similar procedure is followed in electrolytic zinc plants, where electrolyte is sometimes purged from the system because of a build up of manganese impurities. The zinc concentration in the solution is reduced in so-called "stripping" cells prior to final neutralization and disposal. Unfortunately, with this technique it is only possible to reduce the zinc concentration to a level of 10-20 g/L, so that a considerable quantity of zinc remains in solution.
There are a number of difficulties with this process:
As the metal is deposited at the cathode of the electrowinning cell, acid is generated at the anode and the pH of the solution drops. While copper can be plated from highly acidic solutions to a relatively low concentration, other metals such as nickel, zinc and iron will not electroplate efficiently under highly acidic conditions. To avoid this problem with these other metals, it is necessary to continuously adjust the pH upwards through addition of an alkali such as sodium hydroxide. The cost of the alkali consumed in this process is appreciable and for that reason this process is unattractive.
As the concentration of metal decreases in the electrowinning solution, the rate at which metal ions diffuse to the surface of cathode decreases, as there is less driving force for this diffusion. For a given current density, eventually a point is reached where the rate of metal deposition at the cathode is greater than the rate of metal diffusion to the surface of the cathode and concentration of metal in the solution immediately adjacent to the cathode will decrease well below the concentration in the bulk solution. This phenomenon is known as concentration polarization. Concentration polarization results in a deterioration of the quality of the deposit. Instead of a smooth adhering deposit, a powdery, poorly adhering deposit is produced. In some cases, dendrites will grow from the cathode and short circuit against the anode. In addition the cathodic current efficiency will be reduced as hydrogen gas begins evolving instead of .metal deposition. To avoid concentration polarization it is necessary to reduce the current density in proportion to the reduction in the metal concentration. This increases the size of the electrowinning cell required to remove a given quantity of metal. It is not in fact practical to reduce the metal concentration to the level where the solution would be acceptable for discharge to the environment and supplemental conventional precipitation treatment of the solution after electrowinning would be necessary. For this reason only a portion of the metal can be recovered.
If the plating solution contains significant quantities of chloride ions, chlorine gas will evolve at the insoluble anode in addition to, or instead of oxygen. Chloride ions are very corrosive to most common insoluble anode materials, such as lead, and chlorine gas is highly toxic. Provision must be made in the design of the electrowinning cell to handle the chlorine gas generated.
Thus it can be readily seen that use of an electrowinning cell in the usual manner is not a viable means of dealing with the problem of metal concentration buildup in the electroplating bath.
The Brown '572 patent (supra) teaches one way to solve the problem of increasing metal concentration in the electroplating bath. By this method a small percentage of the soluble anodes in the electroplating bath are replaced with insoluble anodes. The quantity of insoluble anode material is selected so that the overall anode efficiency in terms of metal dissolution is equal to the cathode metal deposition efficiency. Consequently, metal dissolves from the anodes at the same rate that it deposits on the cathodes and no buildup occurs.
In some soluble anode plating processes it is not feasible to install a small percentage of insoluble anode material as taught in the aforementioned patent. For example, in the so-called "electro-galvanizing" process, strip steel is continuously plated with metals such as zinc, zinc-iron, or zinc-nickel alloy. The current densities are extremely high and the geometry of the anode is critical to achieving an even current distribution at low operating voltage.